The invention is directed to a circuit arrangement for the automatic monitoring of protection time periods in street traffic signal systems, in which, upon the commencement of the "stop" signal for a group of signals, a switching device determining the protection time for one or more incompatible groups of signals is initiated.
A circuit of this general type is disclosed in German Pat. No. 1,132,835, in which the signal contacts are arranged in series with contacts of local timed switching mechanisms each of which respond after the disconnection of the signal lamps releasing the traffic in one traffic direction and, for the duration of their switching delay which is governed by the clearance time of the intersection, suppress or delay the connection of the signal lamps releasing traffic in the other traffic direction. In such known device, the time switching mechanisms are in the form of electric motors which are actuated at the beginning of the protection time and are automatically stopped by a cam switch at the end of the protection time. Timed mechanisms of this type are relatively expensive and require a relatively large amount of space. Further, such a switching mechanism, employing a cam switch, can in each case represent only a single specific protection time. So that in general, a group of signals would have to have a special motor for each incompatible traffic direction in order to be able to provide the respective different time clearance priods.
Modern traffic signal devices frequently are controlled from a central location or device in order to permit arbitrary variations in the signal operational sequence, on the basis of superordinate considerations, without the necessity of any switch-overs or changes at the relevant street intersection. Thus, in such cases each signal group obtains its "stop" and "go" (red and green) command signals from the central switching device, whereby the switching sequence cannot be controlled at the intersection itself. This, however, creates the dangerous possibility that in the event of an incorrect signal transmission on the lines, which often may be quite long, dangerous situations can arise, i.e. when incompatible traffic flows simultaneously receive a "go" signal or when a "go" signal arrives too early and the necessary protection time periods are not assured.